Neutron stars will tell us if they’re just an illusion

About 100 years ago, general relativity was very successful in describing gravity on a variety of systems, passing all experimental tests on Earth and in the Solar System. However, to explain cosmological observations such as the observed accelerating expansion of the universe, we must introduce dark components, such as dark matter and dark energy, which remain a mystery.

Enrico Barausse, an astrophysicist at SISSA (Scuola Internazionale Superiore di Studi Avanzati) and principal investigator for the ERC GRAMS (GRavity from Astrophysical to Microscopic Scales) grant wonders whether dark energy is real or, conversely, if it can be interpreted as A break in our understanding of gravity. “The presence of dark energy could be an illusion,” he says, “which could be caused by the accelerating expansion of the universe due to hitherto unknown changes in general relativity, a kind of ‘dark gravity’.”

Merging neutron stars provides a unique case for testing this hypothesis because the gravity around them is being pushed to the extreme. The scientist explains that “neutron stars are the densest stars in existence, usually with a radius of only 10 km, but their mass is more than or twice the mass of our Sun.” This makes the gravity and spacetime around them intense, allowing for copious production of gravitational waves when two of them collide. We can use data obtained from such events to study how gravity works and test Einstein’s theory in a new window. »

In this study published in physical examination lettersSISSA scientists, in collaboration with physicists from Universidade de les Ballers in Palma de Mallorca, have produced the first simulations of a binary merger between neutron stars in cosmologically relevant modified gravitational theories: “This kind of simulation is very challenging,” outlines Miguel Pezares, first author For the article, “Because of the nonlinear nature of the problem. This requires an enormous computational effort – months of operation in supercomputers – which was also made possible by the agreement between SISSA and the CINECA consortium as well as the new mathematical formulas we developed. These were significant hurdles for many years until the first simulations we’ve got. “

Thanks to this simulation, researchers can finally compare general relativity and modified gravity. “Amazingly, we found the ‘dark gravity’ hypothesis to be as good as general relativity at explaining data obtained by LIGO and Virgo interferometers from previous neutron star binary collisions. In fact, the differences between the two theories in these systems are very subtle., but They can be detected by next-generation gravitational interferometers, such as Einstein telescope in Europe and cosmic explorer in the United States of America. Baraus concludes that this opens up the exciting possibility of using gravitational waves to distinguish dark energy from “dark gravity”.

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